1. Field of the Invention
The present invention relates to high density memory devices based on phase change memory materials, like chalcogenides, and on other programmable resistance materials, and methods for manufacturing such devices.
2. Description of Related Art
Phase-change-based memory materials, like chalcogenides and similar materials, can be caused to change between an amorphous phase and a crystalline phase by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous phase is characterized by higher electrical resistivity than the generally crystalline phase, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistance material to form nonvolatile memory circuits, which can be read and written with random access.
It is desirable to reduce the cross-sectional area or footprint of individual memory cells in an array in order to achieve higher density memory devices. In programmable resistance devices, a limiting factor in the footprint can be the layout of access devices used in the array to select individual cells. One approach to reducing the footprint of access devices in memory arrays has been based on the development of vertical transistors, where the source, channel and drain (or emitter, base and collector) arranged in a stack and extend through a word line or other gate conductor in a manner that aligns the gate with the middle terminal. See for example, commonly owned, co-pending U.S. patent application Ser. No. 12/471,287, filed: 22 May 2009 (MXIC 1854-1) which is incorporated by reference as if fully set forth herein. However, limitations on the footprint are imposed by the multiple patterning steps required for manufacturing the vertical transistors and the memory elements in contact with them.
It is therefore desirable to provide memory cells having vertical transistor access devices for use in high-density memory devices suitable for use in programmable resistance devices.